1. Field of the Invention
The present invention relates to compositions and methods for inhibiting the degradation of hydroxyalkylidene phosphonate scale inhibitors by the oxidizing biocides bromine and chlorine in an aqueous system where said phosphonates have been added to prevent the formation, deposition and adherence of alkaline earth metal scale deposits, especially calcium carbonate (CaCO.sub.3) scale deposits, on metallic surfaces of said aqueous systems, e.g., cycled up recirculating cooling systems, where the biocide, most likely bromine, has been added to said aqueous system in order to control the growth of microbes in said system.
In order for the hydroxyalkylidene phosphonate scale inhibiting compositions described further below to prevent the formation and deposition of scale in such an aqueous system, they must maintain a level of resistance to degradation by the oxidizing biocide, especially bromine. This is of particular importance in cooling systems such as those using cycled up recirculating cooling towers, where bromine is usually used in preference to chlorine.
Such systems maintain a large body of water for a considerable length of time exposed to the atmosphere under conditions which do not include sufficient aeration and exposure to sunlight to provide control of microbial, especially bacterial and fungal, growth. In particular, many cooling towers use fill composed of beads of synthetic polymer or other materials, in order to extend the amount of heat exchange surface area, and this type of construction greatly aggravates the problem of microbiological growth, since it provides an ideal physical environment for the propagation of troublesome microbes. Unchecked, such microorganisms flourish and produce colonies extensive enough to give rise to problems of biofilm blockage of heat exchange surfaces, as well as clogging of the components of the water transporting apparatus used in operating the cooling system.
As already indicated, such problems of unwanted microbial growth in a cooling system are usually solved by use of an oxidizing biocide, especially chlorine or bromine, since these are inexpensive, effective, and produce minimal environmental impact. However, as is well known, such oxidizing biocides also tend to degrade scale inhibitors containing a hydroxyalkylidene phosphonate group, presumably by oxidative attack in which the hydroxy group plays a role, with resultant cleavage through the alkylidene group and release of phosphate PO.sub.4.sup.-3 ions. This attack on the hydroxyalkylidene phosphonate scale inhibitor produces detrimental results in three ways. First, there is a loss of the phosphonate scale inhibitor product itself, with consequent scale buildup. Second the PO.sub.4.sup.-3 ions which are released react with calcium cations to form calcium phosphate scale, a very hard and intractable scale. Third, the PO.sub.4.sup.-3 ions also adversely modify the adherence properties of the calcium carbonate scale whose formation and deposition the phosphonate scale inhibitor was intended to prevent in the first place.